The definition of Art has two parts. These two things can be talked about seperately but in Art are always found together. Like heads or tails of a coin, it is difficult to define one part without making some reference to the other, (they do not exist seperately, they are part of the same).

Form: Form is the shape the art takes, it's physical nature, i.e. Paint, Clay, etc.
Content: Content is the subject of the Art, or the reason that the Art exists, i.e. Portrait of a face, Landscape, etc.

The realtionship between the two parts determines the quality of the work, and how it is understood.
Form
Content

Drawing

Paper, Pencil,Pastels, CrayonSketchingCartoonsGraffitti
Portrait

Sculpture

Clay,Origami, Paper Mache, StoneWood carvingCementIce
Sand

Painting

Acrylic,Oil, Water colour, Spray PaintFinger Painting

Photography

Digital, Colour Film, Black and White Film, Pin Hole Cameras

Movies

Animation, Claymation, Live Action, Television

Magazines/Books

Comic books, Art books/cataloguesFashion magazines

Collage

Mosaic tile, Construction paper, Magazines, Photographs, Cloth
People===Animals=== ===Abstract=== ===Batman=== ===Superman=== ===Emotions=== ===Landscape=== ===Beliefs=== ===Shades/Colours=== ===Time Periods=== ===Plants=== ===Earth=== ===Vikings=== ===Vehicles=== ===Ghosts=== ===Aliens=== ===Regions=== ===Anything the imagination can imagine===